This invention is an improved servo system and pertains more particularly to equipment for controlling the motion of a plurality of axes in a numerically controlled machine tool.
Servo systems are widely used in the control of machinery where the rotational or translational position of a commanded element is controlled by the corresponding movement of a control element. Typically, a feedback transducer monitors the motion of the commanded element and sends back to a summing junction an indication of the actual position. The summing junction compares the actual positon with the commanded position, generating a position error which is used to correct the system.
A system of this type would be used, for instance, in numerically controlled machine tools like lathes and milling machines where data read from a paper tape would be used to command the positioning of a part being manufactured in a machine tool or of a cutting tool which is being used by a machine tool. Typically, the data read from the tape would specify the starting point, end point and path of said tool or part. Concurrently a position feedback transducer would generate a tool position signal which would be compared to the commanded position thus generating a position error and a corrected new position command, closing the servo loop. In this way a part could be produced on the machine tool that is an exact representation of the part commanded by the paper tape instructions.
The position feedback transducers usually are resolvers and comprise two sets of stator coils spatially placed 90.degree. apart and driven by electrical signals that are themselves 90.degree. apart (sin .omega.t and cos .omega.t). A rotor is rotatably attached to the axis and rotates so that it will pick up from the stators an output sine wave varying in phase (sin .omega.t + .phi.). Thus, the output of a resolver varies in phase as the axis transducer varies in rotational position. This phase difference must then be converted into a voltage level before being sent to the summing junction.
Because of the mechanical linkages and voltage and phase comparisons, position control systems of this kind are relatively large, complex, and expensive, particularly when multiple degrees-of-freedom systems are to be controlled, each requiring its own complete control and servo system. Maintenance and adjustment are also difficult, particularly where great accuracy is sought, as in numerical control machining systems. What is needed in a numerical control machining system is a low cost and reliable position control system that may easily be calibrated to a high degree of accuracy.